Traditionally, OCTG and the like have been used by connecting end portions of pipes thereof with threads formed at those end portions of pipes. Such threads have been subjected to various improvements such as in the thread shape in association with demands for deepening of oil wells, and higher resistance to corrosive environments (see, for example, Non Patent Literature 1). Such threads, which may possibly be formed at an end portion of a pipe having a length of several tens of meters and a weight of as large as several hundreds of kgf, have a complex and high precision thread shape. Meanwhile, regarding such threads, quality control items called thread elements are defined, and whether or not a measured value of a thread element is within a predetermined tolerance is inspected. Examples of the thread element include an outer diameter of the thread part, an outer diameter of the sealing part, an outer diameter of the parallel part, a diameter of the thread groove, a height of the thread ridge, a depth of the thread groove, a thread taper, a seal taper, and the like.
While, conventionally, thread elements, which are the quality control items as described above, have been manually measured on-line (on a thread processing line) by use of a special purpose measurement instrument, attempts have been made to develop more precise automatic measurement technique in the view point of labor saving, suppression of human errors, and enhancement of the speed and precision of measurement.
Specifically, as a technique for automatically measuring thread elements, there is known an automatic measurement apparatus which has an optical sensor, which projects parallel light from a light source onto thread grooves and detects the light that leaks out to the opposite side of the light source with respect to the pipe axis, and measures thread elements based on the detection result of the optical sensor (see, for example, Patent Literatures 1 and 2).